Hot melt adhesives are generally a solid thermoplastic material which quickly melts upon heating and then sets to a firm bond on cooling. In contrast to most other types of adhesives which typically set by evaporation of a solvent or aqueous medium, hot melt adhesives are generally homogeneous mixtures. In the manufacture of hot melt adhesives, the thermoplastic material is typically compounded with one or more tackifiers, waxes, antioxidants and/or other stabilizing additives, and may be cooled for later use. The hot melt adhesive is typically heated in a reservoir from which it is supplied to a substrate by pumping or pressure extrusion through nozzles or slot dies. In contrast to the solvent-based or emulsion-type adhesives which require time for the evaporation of the solvent or aqueous medium, an almost instantaneous bonding can be obtained with hot melt adhesives.
A typical class of hot melt adhesive composition utilizes polyolefins as the base or carrier material. While polyolefins have experienced tremendous growth in their use as hot melts, their utilization has nevertheless been limited by several serious shortcomings. Polyolefin-based hot melts, for example, usually suffer from a lack of strong adhesion to non-porous metallic surfaces, such as aluminum, steel and the like. They also tend to be quite brittle unless suitably modified. All hot melts generally suffer from low bond strengths at elevated temperatures. When heated, even well below their melting points, they tend to soften and lose strength.
In the automotive industry in structural and semi-structural applications, the low temperature brittleness and high temperature softening difficulties of previous polyolefin hot melts have been approached by changing to more expensive epoxy adhesives to which are added elastomers to make rubber-toughened epoxies. However, the rubber-toughened epoxies have in turn suffered from a disadvantage in that low molecular weight materials remain in the epoxy material which are leached out into bathing solutions and these solutions must be changed out more frequently, often at great expense, because such low molecular weight constituents from the epoxy resins build up and accumulate therein. It would be advantageous if the less expensive olefin polymers could be obtained having similar properties to the epoxies, but without the presence of low molecular weight materials typically associated with the epoxy resins. As far as we are aware, however, no technology has heretofore provided such advantages.
From U.S. Pat. Nos. 3,892,819 and Re. 31,310, both to Najvar, it is known to react a polyepoxide with an unsaturated monocarboxylic acid monomer and a liquid carboxy terminated elastomer to form a terminally unsaturated vinyl ester resin which can be cured in admixture with a copolymerizable monomer by polymerization of the vinyl moieties in the presence of a peroxide curing agent. U.S. Pat. No. 3,928,419 to Waters is similar.
Aqueous emulsions of a poly(ethyl acrylate), a copolymer of ethylene and acrylic acid, and an epoxy resin, each component being present within critical concentration limits, is described in U.S. Pat. No. 3,905,931 to Ziegert.
From U.S. Pat. No. 3,912,773 to Havriliak, it is known to use a casting resin system wherein a vinyl polymerization reaction product containing acid functionalities is cured via an acid-epoxide reaction with a diepoxide. The various reactants are prepared in two parts so that the acid monomer and the diepoxy are separated until just prior to use to avoid premature reaction.
From U.S. Pat. Nos. 4,021,504 to Conrad et al., and 4,067,926 to Cadman et al., it is known to use powder coating compositions containing mixtures of a copolymer of an olefinically unsaturated carboxylic acid and an ethylenically unsaturated ester, and a di- or multifunctional epoxide curing agent. The powder coating is prepared by mixing powders of the copolymer and the epoxide, or by melt blending the copolymer and the epoxide and forming the mixture into a powder. The powder is applied to the surface of an article to be coated therewith, and cured by thermosetting at elevated temperature.
From U.S. Pat. No. 4,102,942 to Smith et al., it is known to prepare a high solid composition comprising a blend of low molecular weight acrylic copolymers having pendant carboxyl groups and aliphatic polyepoxides.
From U.S. Pat. No. 4,612,349 to Nicco et al., it is known to use a crosslinking process wherein an ethylene-maleic anhydride-unsaturated carboxylic acid ester terpolymer is reacted with a polyepoxide added to the terpolymer just prior to application and which gradually crosslinks the terpolymer after being cooled.
U.S. Pat. No. 4,806,578 to Kobayashi et al. describes a highly absorptive resin produced by crosslinking a hydrophilic polymer having carboxylic or carboxylate groups with a polyglycidyl ether having 4 or more epoxy groups, in the presence of water.
From U.S. Pat. No. 4,332,713 to Lehmann, it is known to use a pasty thermosetting adhesive containing a liquid epoxide resin, a latent epoxide curing agent which is insoluble in the epoxide resin, and an insoluble thermoplast powder such as ethylene-acrylic acid-acrylate terpolymer.
In U.S. Pat. No. 4,829,124 to Clark, a thermoplastic elastomer is described which comprises a blend of a carboxylated butadiene-acrylonitrile elastomer and a partially neutralized ethylene-acrylic acid copolymer which has been dynamically crosslinked with a polyfunctional epoxy crosslinking agent.
From U.S. Pat. No. 2,447,367 to Rust et al., it is known to prepare polymeric rosin esters by heating an ester of a rosin and a polyhydric alcohol with a polymer containing carboxyl or carbalkoxy groups. Similar polymeric esters are known from U.S. Pat. No. 3,438,918 to Arlt, Jr. et al. to be used for building tack in ethylene-propylene terpolymer rubbers.
From U.S. Pat. No. 4,137,364 to Ball, III et al., it is known to prepare ethylene-vinyl acetate-vinyl alcohol terpolymers crosslinked with either a silane or a biscaprolactam.
From U.S. Pat. No. 4,430,479 to Merton et al., it is known to prepare a hot melt adhesive comprising a hydroxyl-containing ethylene copolymer and an isocyanate crosslinking agent.
Various art is available teaching hot melts prepared with ethylene and/or vinyl based polymers, such as, for example, U.S. Pat. Nos. 4,322,326 to Pile; 4,325,853 to Acharya et al.; 4,535,013 to Kuhn; 4,602,056 to Waniczek et al.; 4,618,640 to Tsuchida et al.; 4,622,357, also to Tsuchida et al.; and 4,631,308 to Graham et al.